Buying 2x HD6870 for 11k each, you think each will sell for 9k?
Yes, when HD 6900 series was launched I was thinking to get a HD 6950 and to unlock it to HD 6970 and selling off my existing XFX HD 6870. I got a quote of 9K for it. I think still I can sell it to 8K.
And that's not the point, using SLI or Crossfire has its own benefits:-
1. You will get better performance than today's high end single GPU based cards.
2. You will do it in a cheaper price and can save a good amount of money.
3. With driver updates, multi GPU scaling is also getting increased.
4. Regarding selling of your GPU, lets say you have a GTX 580 @ 30K. I guess when you have spent 30K, then at least it should have a life span of minimum 1.5 to 2 years for an average user (not considering millionaires). Now after that much of time going multi GPU with such an old but costly card is not viable. So you have to sell it in a far cheaper price.
In case of say, two HD6870, in the 1st place, you are saving around 8/9K and You are getting more than 10% better performance than a GTX 580. After two years, you can still sell them around 5 to 6K. I don't wanna start a debate here about what will be the price of your old GPU after 2 years but if you look at different forums' Bazzar section, you will find out that people are selling cards like GTX 275 (original price was 12 to 14K) at 6K.